In the field of paper production and in the field of producing non-woven materials using the technique of bonding with jets of water, the webs tend to emerge damp after the forming and consolidation phase. In fact:                the production of paper requires mixing paper fibre with water in order to feed it onto a conveyor for forming and to encourage the bonding of the fibrous sheet by hydrogen bonds.        the method of bonding with jets of water uses pressurised jets of water for intertwining the fibres and therefore dampens the fibrous sheet in this way.        
In said methods, the webs produced need to be dried by evaporation before they can be spooled.
To achieve this, it is common to use ovens with traversing air (OMEGA or flat type) to perform said drying operation in a production line.
FIG. 1 shows a block diagram of an oven with traversing air.
The web to be dried 1 circulates round a drum 2 (or on a conveyor in the case of a flat oven).
Pressurised hot air is injected into the hood 3 by means of a fan V1 6 (referred as the main fan) and a heat source 4 which heats the air. Said heat source 4 can be for example a gas burner or a heat exchanger (using oil, air or water).
The temperature of the hot air injected into the hood 3 is controlled by the influence of the heat source 4.
Said hot air then traverses the damp web and the drum 2 (or the conveyor in the case of a flat oven): by means of this method the water contained in the web is evaporated as the web advances on the drum 2 (or on the conveyor in the case of a flat oven). The air which has traversed the web is cooled and is charged with humidity. It is then suctioned inside of the drum 2 (or onto the conveyor in the case of a flat oven) by the fan V1 6, then re-heated by the heat source 4 and re-injected into the circuit and so forth.
The heat source 4 can be placed upstream or downstream of the fan V1 6. Preferably, it is placed upstream in the case of a gas burner and downstream in the case of a heat exchanger.
A portion of the cold and humid air needs to be evacuated outside of the circuit so as not to concentrate the humidity in the circulation circuit. Therefore, a portion of the cold and humid air is evacuated (5 to 30% of the total flow) by the branch pipe 5 by means of the pressure generated at the outlet by the fan V1 6 if this is sufficient or by means of an additional branch fan V2 8 if the pressure generated by the fan V1 6 is not sufficient.
The branch pipe can be connected upstream of V1.
The flow of branched air is controlled either by influencing the rotational speed of the fan V2 8 or by influencing the opening of the control flaps 9.
To address this deficit of air in the circuit reserve air from the production hall or outside the building is suctioned upstream of the fan V1 6 via the reserve pipe 7.
In the case of paper production, the air in the hood is usually heated to temperatures of about 200-250° C. The air extracted from the oven leaves at a temperature varying between 150 and 180° C.
In the case of the production of non-woven material which uses the water jet method of bonding, the air in the hood is usually heated to temperatures of about 100-150° C. The air extracted from the oven is evacuated preferably at a temperature varying between 70 and 120° C.
The drying temperatures for producing non-woven material which uses the method of water jet bonding are lower because some fibres forming the non-woven material (POLYPROPYLENE or POLYESTER for example) are sensitive when exposed to very high temperatures (with shrinkage of width of the web, reduction of the thickness of the web, yellowing of the web, alteration of the mechanical properties of the web).
Several improvement devices describe different ways of using the thermal energy from the air which is discharged by the oven through the extraction pipes.
In particular, in the case of paper production one of the improvement devices (cf. patents DE2802156B1 and U.S. Pat. No. 6,551,461) injects the still hot branched air above a drying box located upstream of the oven in order to start the method of drying the web upstream of the oven (and thus reduce the amount removed from the web on entry into the oven).
The branch air is supplied by means of a diffusion box 10 above a drying box 11 on which the web circulates.
Said drying box 11 can be integrated into a conveyor or into a cylinder.
The still hot air supplied in this way traverses the web and thereby evaporates a portion of the water contained in the web.
The air recovered by the drying box 11 (cold and humid) is then evacuated to the outside (by a final extraction fan for example).
The system described above functions perfectly well when the temperature of the air discharged by the oven is greater than 150° C. (in the case of paper production). In fact, at these temperatures, the relative humidity of the current of air which is discharged by the oven remains low (<5%).
During the passage of this current of air through the damp web on the drying box, its temperature drops again and the air is again charged with humidity. However, because of the favourable conditions upstream (raised temperature, low relative humidity), it is not possible for the suctioned air to reach saturation by traversing the web on the drying box and regenerate water on the web by condensation.
In the case of a non-woven web made by a water jet method of bonding, said system for improving the drying described in the patents DE2802156B1 and U.S. Pat. No. 6,551,461 does not function well.
It is noted that the web is dampened rather than dried.